[BUGFIX] EXT:linkvalidator Keep field content when saving new task

When saving a new linkvalidator task in the scheduler and the validation
failed, now the previous input is kept in the external fields.

Resolves: #79056
Releases: master
Change-Id: Ib629f47890f2972de8c8bfa28644ef933357c374
Reviewed-on: https://review.typo3.org/51011


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>